They're .svg files, but seem to not be fully compliant --- changing the file extension to .svg allows them to open in Google's Chrome web browser --- but they won't open in InkScape or Adobe Illustrator last I checked.
There are some tools which have been posted here:
https://www.mobileread.com/forums/showthread.php?t=83529
https://www.mobileread.com/forums/showthread.php?t=67714